Binuclear compounds

ABSTRACT

Devices are provided having an anode, a cathode, and an emissive layer disposed between and electrically connected to the anode and the cathode. The emissive layer includes an emissive material having more than one metal center. In one embodiment, first and second metal centers are independently selected from the group consisting of d7, d8 and d9 metals. A bridging ligand is coordinated to the first metal center and to the second metal center. In one embodiment, the first and second metal centers each have coordination numbers of at least 3, and more preferably each have coordination numners of 4. In one embodiment, photoactive ligands are coordinated to the first and second metal centers. In one embodiment, there are no photoactive ligands. In one embodiment, a charge neutral bi-nuclear emissive material is provided. In one embodiment the first and metal centers have a co-facial configuration, and preferably a square planar co-facial configuration. In one embodiment, the metal centers are selected from metals having an atomic number greater than or equal to 40.

FIELD OF THE INVENTION

[0001] The present invention relates to organic light emitting devices(OLEDs), and more specifically to phosphorescent organo-metallicmaterials used in such devices. More specifically, the present inventionrelates to OLEDs, wherein the emissive layer comprises a phosphorescentemitting material having a plurality of metal centers.

BACKGROUND

[0002] Opto-electronic devices that make use of organic materials arebecoming increasingly desirable for a number of reasons. Many of thematerials used to make such devices are relatively inexpensive, soorganic opto-electronic devices have the potential for cost advantagesover inorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

[0003] As used herein, the term “organic” includes polymeric materialsas well as small molecule organic materials that may be used tofabricate organic opto-electronic devices. “Small molecule” refers toany organic material that is not a polymer, and “small molecules” mayactually be quite large. Small molecules may include repeat units insome circumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

[0004] OLEDs make use of thin organic films that emit light when voltageis applied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

[0005] OLED devices are generally (but not always) intended to emitlight through at least one of the electrodes, and one or moretransparent electrodes may be useful in an organic opto-electronicdevice. For example, a transparent electrode material, such as indiumtin oxide (ITO), may be used as the bottom electrode. A transparent topelectrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745,which are incorporated by reference in their entireties, may also beused. For a device intended to emit light only through the bottomelectrode, the top electrode does not need to be transparent, and may becomprised of a thick and reflective metal layer having a high electricalconductivity. Similarly, for a device intended to emit light onlythrough the top electrode, the bottom electrode may be opaque and/orreflective. Where an electrode does not need to be transparent, using athicker layer may provide better conductivity, and using a reflectiveelectrode may increase the amount of light emitted through the otherelectrode, by reflecting light back towards the transparent electrode.Fully transparent devices may also be fabricated, where both electrodesare transparent. Side emitting OLEDs may also be fabricated, and one orboth electrodes may be opaque or reflective in such devices.

[0006] As used herein, “top” means furthest away from the substrate,while “bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

SUMMARY OF THE INVENTION

[0007] Devices are provided having an anode, a cathode, and an emissivelayer disposed between and electrically connected to the anode and thecathode. The emissive layer includes an emissive material having morethan one metal center. In one embodiment, first and second metal centersare independently selected from the group consisting of d7, d8 and d9metals. A bridging ligand is coordinated to the first metal center andto the second metal center. In one embodiment, the first and secondmetal centers each have coordination numbers of at least 3, and morepreferably each have coordination numners of 4. In one embodiment,photoactive ligands are coordinated to the first and second metalcenters. In one embodiment, there are no photoactive ligands. In oneembodiment, a charge neutral bi-nuclear emissive material is provided.In one embodiment the first and metal centers have a co-facialconfiguration, and preferably a square planar co-facial configuration.In one embodiment, the metal centers are selected metals having anatomic number greater than or equal to 40.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows an organic light emitting device having separateelectron transport, hole transport, and emissive layers, as well asother layers.

[0009]FIG. 2 shows an inverted organic light emitting device that doesnot have a separate electron transport layer.

[0010]FIG. 3 shows the photoluminescent emission spectrum of a thin filmof (F₂ppy)₂Pt₂(SPy)₂ doped at 5% into a CBP.

[0011]FIG. 4 shows the excitation spectra of a thin film of(F₂ppy)₂Pt₂(SPy)₂ doped at 5% in CBP with emission set at 605 nm. Theabsorption band at 500 nm can be seen.

[0012]FIG. 5 shows a comparison of the emission spectra of FPt, FPtdpmand (F₂ppy)₂Pt₂(SPy)₂. Photoluminescent films of FPt, FPtdpm and(F₂ppy)₂Pt₂(SPy)₂ in CBP, each doped at 5%, were prepared by spincoating and were excited at 370 nm.

[0013]FIG. 6 show a schematic representation of the device ITO/NPD (400Å)/binuclear complex, 9%: CBP (300 Å)/BCP (120 Å, optional)/Zrq₄(350Å)/Li:Al (1000 Å).

[0014]FIG. 7 shows plots of quantum efficiency against current densityfor devices having the structures illustrated in FIG. 6.

[0015]FIG. 8 shows plots of current density vs. voltage for deviceshaving the structures illustrated in FIG. 6.

[0016]FIG. 9 shows plots of electroluminescent spectra for deviceshaving the structures illustrated in FIG. 6.

[0017]FIG. 10 shows plots of brightness vs voltage for the deviceshaving the structures illustrated in FIG. 6.

[0018]FIG. 11 show a schematic representation of the device ITO/NPD (400Å)/mCP (200 Å, optional)/binuclear complex, 9%: mCP (300 Å)/Zrq₄(350Å)/Li:Al (1000 Å).

[0019]FIG. 12 shows plots of quantum efficiency against current densityfor devices having the structures illustrated in FIG. 11.

[0020]FIG. 13 shows plots of current density vs. voltage for deviceshaving the structures illustrated in FIG. 11.

[0021]FIG. 14 shows plots of electroluminescent spectra for deviceshaving the structures illustrated in FIG. 11.

[0022]FIG. 15 shows plots of brightness vs voltage for the deviceshaving the structures illustrated in FIG. 11.

[0023]FIG. 16 shows the chemical structure of FPt, FPtdpm, and(F₂ppy)₂Pt₂(SPy)₂.

[0024]FIG. 17 shows the structure of (F₂ppy)₂Pt₂(SPy)₂ as determined byX-ray crystallography.

[0025]FIG. 18 shows PL emission spectra for Pt₂Spy₄ in solution

[0026]FIG. 19 shows absorption spectra for Pt₂Spy₄ and Spy in solution.

DETAILED DESCRIPTION

[0027] Generally, an OLED comprises at least one organic layer disposedbetween and electrically connected to an anode and a cathode. As usedherein, the term “disposed between and electrically connected to” doesnot indicate that the recited layers are necessarily adjacent and indirect contact. Rather, it allows for the disposition of additionallayers between the recited layers. When a current is applied to thedevice, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

[0028] The initial OLEDs used emissive molecules that emitted light fromtheir singlet states (“fluorescence”) as disclosed, for example, in U.S.Pat. No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

[0029] More recently, OLEDs having emissive materials that emit lightfrom triplet states (“phosphorescence” ) have been demonstrated. Baldoet al., “Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-Bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that amaterial that exhibits phosphorescence at liquid nitrogen temperaturesmay not exhibit phosphorescence at room temperature. But, asdemonstrated by Baldo, this problem may be addressed by selectingphosphorescent compounds that do phosphoresce at room temperature.

[0030] Generally, the excitons in an OLED are believed to be created ina ratio of about 3:1, i.e., approximately 75% triplets and 25% singlets.See, Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency InAn Organic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001),which is incorporated by reference in its entirety. In many cases,singlet excitons may readily transfer their energy to triplet excitedstates via “intersystem crossing,” whereas triplet excitons may notreadily transfer their energy to singlet excited states. As a result,100% internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

[0031] Phosphorescence may be preceded by a transition from a tripletexcited state to an intermediate non-triplet state from which theemissive decay occurs. For example, organic molecules coordinated tolanthanide elements often phosphoresce from excited states localized onthe lanthanide metal. However, such materials do not phosphorescedirectly from a triplet excited state but instead emit from an atomicexcited state centered on the lanthanide metal ion. The europiumdiketonate complexes illustrate one group of these types of species.

[0032] Phosphorescence from triplets can be enhanced over fluorescenceby confining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

[0033]FIG. 1 shows an organic light emitting device 100. The figures arenot necessarily drawn to scale. Device 100 may include a substrate 110,an anode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

[0034] Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

[0035] Anode 115 may be any suitable anode that is sufficientlyconductive to transport holes to the organic layers. The material ofanode 115 preferably has a work function higher than about 4 eV (a “highwork function material”). Preferred anode materials include conductivemetal oxides, such as indium tin oxide (ITO) and indium zinc oxide(IZO), aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate110) may be sufficiently transparent to create a bottom-emitting device.A preferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. Anode 115 may be opaque and/or reflective. A reflective anode115 may be preferred for some top-emitting devices, to increase theamount of light emitted from the top of the device. The material andthickness of anode 115 may be chosen to obtain desired conductive andoptical properties. Where anode 115 is transparent, there may be a rangeof thickness for a particular material that is thick enough to providethe desired conductivity, yet thin enough to provide the desired degreeof transparency. Other anode materials and structures may be used.

[0036] Hole transport layer 125 may include a material capable oftransporting holes. Hole transport layer 130 may be intrinsic (undoped),or doped. Doping may be used to enhance conductivity. α-NPD and TPD areexamples of intrinsic hole transport layers. An example of a p-dopedhole transport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of50: 1, as disclosed in U.S. patent application Ser. No. 10/173,682 toForrest et al., which is incorporated by reference in its entirety.Other hole transport layers may be used.

[0037] Emissive layer 135 will comprise at least one emissive materialcapable of emitting light when a current is passed between anode 115 andcathode 160. Preferably, emissive layer 135 contains a phosphorescentemissive material, although fluorescent emissive materials may also beused. Phosphorescent materials are preferred because of the higherluminescent efficiencies associated with such materials. Emissive layer135 may also comprise a host material capable of transporting electronsand/or holes, doped with an emissive material that may trap electrons,holes, and/or excitons, such that excitons relax from the emissivematerial via a photoemissive mechanism. Emissive layer 135 may comprisea single material that combines transport and emissive properties.Whether the emissive material is a dopant or a major constituent,emissive layer 135 may comprise other materials, such as dopants thattune the emission of the emissive material. Emissive layer 135 mayinclude a plurality of emissive materials capable of, in combination,emitting a desired spectrum of light. Thus, in one embodiment of thepresent invention, the emissive layer comprises a binuclear emissivematerial and a second emissive material, such that the combined emissionsufficiently spans the visible spectrum to give a white emission.Examples of fluorescent emissive materials include DCM and DMQA.Examples of host materials include Alq3, CBP and mCP. Examples ofemissive and host materials are disclosed in U.S. Pat. No. 6,303,238 toThompson et al., which is incorporated by reference in its entirety.Emissive material may be included in emissive layer 135 in a number ofways. For example, an emissive small molecule may be incorporated into apolymer. Other emissive layer materials and structures may be used.

[0038] Electron transport layer 140 may include a material capable oftransporting electrons. Electron transport layer 140 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1: 1, as disclosed in U.S. patent application Ser. No. 10/173,682 toForrest et al., which is incorporated by reference in its entirety.Other electron transport layers may be used.

[0039] The charge carrying component of the electron transport layer maybe selected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) level of theelectron transport layer. The “charge carrying component” is thematerial responsible for the LUMO that actually transports electrons.This component may be the base material, or it may be a dopant. The LUMOlevel of an organic material may be generally characterized by theelectron affinity of that material and the relative electron injectionefficiently of a cathode may be generally characterized in terms of thework function of the cathode material. This means that the preferredproperties of an electron transport layer and the adjacent cathode maybe specified in terms of the electron affinity of the charge carryingcomponent of the ETL and the work function of the cathode material. Inparticular, so as to achieve high electron injection efficiency, thework function of the cathode material is preferably not greater than theelectron affinity of the charge carrying component of the electrontransport layer by more than about 0.75 eV, more preferably, by not morethan about 0.5 eV. Most preferably, the electron affinity of the chargecarrying component of the electron transport layer is greater than thework function of the cathode material. Similar considerations apply toany layer into which electrons are being injected.

[0040] Cathode 160 may be any suitable material or combination ofmaterials known to the art, such that cathode 160 is capable ofconducting electrons and injecting them into the organic layers ofdevice 100. Cathode 160 may be transparent or opaque, and may bereflective. Metals and metal oxides are examples of suitable cathodematerials. Cathode 160 may be a single layer, or may have a compoundstructure. FIG. 1 shows a compound cathode 160 having a thin metal layer162 and a thicker conductive metal oxide layer 164. In a compoundcathode, preferred materials for the thicker layer 164 include ITO, IZO,and other materials known to the art. U.S. Pat. Nos. 5,703,436 and5,707,745, which are incorporated by reference in their entireties,disclose examples of cathodes including compound cathodes having a thinlayer of metal such as Mg:Ag with an overlying transparent,electrically-conductive, sputter-deposited ITO layer. The part ofcathode 160 that is in contact with the underlying organic layer,whether it is a single layer cathode 160, the thin metal layer 162 of acompound cathode, or some other part, is preferably made of a materialhaving a work function lower than about 4 eV (a “low work functionmaterial”). Other cathode materials and structures may be used.

[0041] Blocking layers may be used to reduce the number of chargecarriers (electrons or holes) and/or excitons that leave the emissivelayer. An electron blocking layer 130 may be disposed between emissivelayer 135 and the hole transport layer 125, to block electrons fromleaving emissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 140.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and U.S. patent applicationSer. No. 10/173,682 to Forrest et al., which are incorporated byreference in their entireties.

[0042] Generally, injection layers are comprised of a material that mayimprove the injection of charge carriers from one layer, such as anelectrode or an organic layer, into an adjacent organic layer. Injectionlayers may also perform a charge transport function. In device 100, holeinjection layer 120 may be any layer that improves the injection ofholes from anode 115 into hole transport layer 125. CuPc is an exampleof a material that may be used as a hole injection layer from an ITOanode 115, and other anodes. In device 100, electron injection layer 150may be any layer that improves the injection of electrons into electrontransport layer 145. LiF/Al is an example of a material that may be usedas an electron injection layer into an electron transport layer from anadjacent layer. Other materials or combinations of materials may be usedfor injection layers. Depending upon the configuration of a particulardevice, injection layers may be disposed at locations different thanthose shown in device 100. More examples of injection layers areprovided in U.S. patent application Ser. No. 09/931,948 to Lu et al.,which is incorporated by reference in its entirety. A hole injectionlayer may comprise a solution deposited material, such as a spin-coatedpolymer, e.g., PEDOT:PSS, or it may be a vapor deposited small moleculematerial, e.g., CuPc or MTDATA.

[0043] A hole injection layer (HIL) may be used to planarize or wet theanode surface as well as to provide efficient hole injection from theanode into the hole injecting material. A hole injection layer may alsohave a charge carrying component having HOMO (Highest Occupied MolecularOrbital) energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO that actually transports holes.This component may be the base material of the HIL, or it may be adopant. Using a doped HIL allows the dopant to be selected for itselectrical properties, and the host to be selected for morphologicalproperties such as wetting, flexibility, toughness, etc. Preferredproperties for the HIL material are such that holes can be efficientlyinjected from the anode into the HIL material. In particular, the chargecarrying component of the HIL preferably has an IP not more than about0.7 eV greater that the IP of the anode material. More preferably, thecharge carrying component has an IP not more than about 0.5 eV greaterthan the anode material. Similar considerations apply to any layer intowhich holes are being injected. HIL materials are further distinguishedfrom conventional hole transporting materials that are typically used inthe hole transporting layer of an OLED in that such HIL materials mayhave a hole conductivity that is substantially less than the holeconductivity of conventional hole transporting materials. The thicknessof the HIL of the present invention may be thick enough to helpplanarize or wet the surface of the anode layer. For example, an HILthickness of as little as 10 nm may be acceptable for a very smoothanode surface. However, since anode surfaces tend to be very rough, athickness for the HIL of up to 50 nm may be desired in some cases.

[0044] A protective layer may be used to protect underlying layersduring subsequent fabrication processes. For example, the processes usedto fabricate metal or metal oxide top electrodes may damage organiclayers, and a protective layer may be used to reduce or eliminate suchdamage. In device 100, protective layer 155 may reduce damage tounderlying organic layers during the fabrication of cathode 160.Preferably, a protective layer has a high carrier mobility for the typeof carrier that it transports (electrons in device 100), such that itdoes not significantly increase the operating voltage of device 100.CuPc, BCP, and various metal phthalocyanines are examples of materialsthat maybe used in protective layers. Other materials or combinations ofmaterials may be used. The thickness of protective layer 155 ispreferably thick enough that there is little or no damage to underlyinglayers due to fabrication processes that occur after organic protectivelayer 160 is deposited, yet not so thick as to significantly increasethe operating voltage of device 100. Protective layer 155 may be dopedto increase its conductivity. For example, a CuPc or BCP protectivelayer 160 may be doped with Li. A more detailed description ofprotective layers may be found in U.S. patent application Ser. No.09/931,948 to Lu et al., which is incorporated by reference in itsentirety.

[0045]FIG. 2 shows an inverted OLED 200. The device includes a substrate210, a cathode 215, an emissive layer 220, a hole transport layer 225,and an anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

[0046] The simple layered structure illustrated in FIGS. 1 and 2 isprovided by way of non-limiting example, and it is understood thatembodiments of the invention may be used in connection with a widevariety of other structures. The specific materials and structuresdescribed are exemplary in nature, and other materials and structuresmay be used. Functional OLEDs may be achieved by combining the variouslayers described in different ways, or layers may be omitted entirely,based on design, performance, and cost factors. Other layers notspecifically described may also be included. Materials other than thosespecifically described may be used. Although many of the examplesprovided herein describe various layers as comprising a single material,it is understood that combinations of materials, such as a mixture ofhost and dopant, or more generally a mixture, may be used. Also, thelayers may have various sublayers. The names given to the various layersherein are not intended to be strictly limiting. For example, in device200, hole transport layer 225 transports holes and injects holes intoemissive layer 220, and may be described as a hole transport layer or ahole injection layer. In one embodiment, an OLED may be described ashaving an “organic layer” disposed between a cathode and an anode. Thisorganic layer may comprise a single layer, or may further comprisemultiple layers of different organic materials as described, forexample, with respect to FIGS. 1 and 2.

[0047] Structures and materials not specifically described may also beused, such as OLEDs comprised of polymeric materials (PLEDs) such asdisclosed in U.S. Pat. No. 5,247,190, Friend et al., which isincorporated by reference in its entirety. By way of further example,OLEDs having a single organic layer may be used. OLEDs may be stacked,for example as described in U.S. Pat. No. 5,707,745 to Forrest et al,which is incorporated by reference in its entirety. The OLED structuremay deviate from the simple layered structure illustrated in FIGS. 1 and2. For example, the substrate may include an angled reflective surfaceto improve out-coupling, such as a mesa structure as described in U.S.Pat. No. 6,091,195 to Forrest et al., and/or a pit structure asdescribed in U.S. Pat. No. 5,834,893 to Bulovic et al., which areincorporated by reference in their entireties.

[0048] Unless otherwise specified, any of the layers of the variousembodiments may be deposited by any suitable method. For the organiclayers, preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

[0049] Devices fabricated in accordance with embodiments of theinvention may be incorporated into a wide variety of consumer products,including flat panel displays, computer monitors, televisions,billboards, lights for interior or exterior illumination and/orsignaling, heads up displays, fully transparent displays, flexibledisplays, laser printers, telephones, cell phones, personal digitalassistants (PDAs), laptop computers, digital cameras, camcorders,viewfinders, micro-displays, vehicles, a large area wall, theater orstadium screen, or a sign. Various control mechanisms may be used tocontrol devices fabricated in accordance with the present invention,including passive matrix and active matrix. Many of the devices areintended for use in a temperature range comfortable to humans, such as18 degrees C. to 30 degrees C., and more preferably at room temperature(20-25 degrees C.).

[0050] The materials and structures described herein may haveapplications in devices other than OLEDs. For example, otheroptoelectronic devices such as organic solar cells and organicphotodetectors may employ the materials and structures. More generally,organic devices, such as organic transistors, may employ the materialsand structures.

[0051] As used herein, “solution processible” means capable of beingdissolved, dispersed, or transported in and/or deposited from a liquidmedium, either in solution or suspension form.

[0052] The term “binuclear” as used herein refers to a complex havingexactly two metal centers. The binuclear compounds of the presentinvention comprise two metal centers, wherein each metal center is boundto at least one bridging ligand that is bound to both of the metalcenters. In one embodiment, each metal center is also bound to at leastone “photoactive” ligand in addition to the at least one bridgingligand. The ligands bound to each metal center may provide that metalcenter with a roughly square planar configuration. In some embodimentsof the invention, there may be more than two metal centers, but suchembodiments would not be described as “binuclear.”

[0053] Emissive materials of embodiments of the present invention maycomprise at least one “bridging ligand.” This ligand is referred to asbridging because it is bound to two different metal centers. Thebridging ligand(s) may be capable of bonding to two metal centers suchthat the two metal centers are in close proximity, typically withinabout 4 Å. Distances of 4 Å and less between the metal centers allow forsignificant overlap of the d-orbitals of the individual metal atoms.Preferably, the two metal centers of a binuclear complex are about 2.5to about 2.8 Å apart. The choice of bridging ligands allows for theadjustment of the distance between the two metal centers. By changingthe bridging ligands, the energy of the emission from the binuclearcomplex can be tuned.

[0054] In some embodiments, two metal centers are bound togther in aco-facial configuration by one or more bridging ligands. The co-facialconfiguration is preferred because the distance between the metalcenters, and therefore the emission spectra of the molecule, is morereadily tunable in a co-facial configuration. For example, dependingupon the choice of bridging ligands, a co-facial configuration couldresult in metal-metal distances of 2 Å or less. Distance this small maybe difficult to achieve with configurations that are not co-facial.“Co-facial” means that there are at least three bonds from each metalcenter that define a plane perpendicular to the axis between the twometal centers. A preferred co-facial configuration is a square planarco-facial configuration, that may be achieved, for example, with d⁸metals. In the square planar configuration, each metal center has fourbonds, all approximately in the same plane, and separated by one anotherby approximately 90 degrees. (F₂ppy)₂Pt₂(SPy)₂, as illustrated in FIGS.10 and 11, is an example of a square planar co-facial binuclear metalcompound.

[0055] Preferably, the metal centers are not first row transitionmetals, and are rather selected from the second row metals and higher onthe periodic table of elements, i.e., metals having an atomic numbergreater than or equal to 40. Metals from the second row and higher havehigher spin orbit coupling which leads to emissive materials having ahigher phosphorescent yield.

[0056] Some embodiments have photoactive ligands coordinated with bothmetal centers. The emission of such embodiments may be tuned by usingthe bridging ligand to achieve and control interaction between the πorbitals of two photoactive ligands coordinated to different metalcenters. It is believed that a significant π-π interaction will occurbetween the two photoactive ligands if they are held together at adistance of 3.5 Å or less.

[0057] It is also preferred, that the bridging ligand is not capable ofacting as a bidentate ligand. Thus, it is preferable that the bridgingligand be selected so that the binding sites preferentially bond to twodifferent metal centers rather than to the same metal center.

[0058] The bridging ligands may be referred to as “ancillary” because itis believed that they may modify the photoactive properties of themolecule, as opposed to directly contributing to the photoactiveproperties. However, it may be possible that the bridging ligand is partof the emissive system. The definitions of photoactive and ancillary areintended as non-limiting theories.

[0059] The bridging ligand provides a stable linkage between the twometal centers of a binuclear emissive compound. The bridging ligand maybe symmetric (i.e., the binding sites to the metal centers are the same)or asymmetric (i.e., the binding sites to the metal centers areinequivalent). Thus, the bridging ligand is a molecule having at leasttwo sites for coordination. Suitable bridging ligands may be chosen fromthose known in the art that are capable of providing a stable binuclearspecies. In this context, the term stable refers to the stability of thebinuclear complex when incorporated into a light emitting device, andespecially during the operation of such a device. Some suitable ligandsare disclosed in: Matsumoto et al., “Organometallic chemistry ofplatinum-blue derived platinumIII dinuclear complexes,” CoordinationChemistry Reviews 231 (2002), pages 229-238 and Tejel et al., “FromPlatinum Blues to Rhodium and Iridium Blues,” Chem. Eur. J. (1999) 5,No. 4, pages 1131-1135; Belitto et al., “Metal-Metal Interactions in OneDimension. 3. Segregated Canted Stacks of Tetrakis (dithioacetato)diplatinum (11),” Inorg. Chem. (1980) 19, pages 3632-3636; Oskui et al.,“Di- and Tripalladium(II) and -platinum(II) Complexes Containing7Amino-1,8-naphthyridin-2-one as a Bridging Ligand—Oxidation of a[Pt3]6+Core to [Pt3]8+, “Eur. J. Inorg. Chem. (1999) 1325-1333; Navarroet al., “Binuclear Platinum(II) Triazolopyrimidine Bridged Complexes.Preparation, Crystal Structure, NMR Spectroscopy, and ab Initio MOInvestigation on the Bonding Nature of the Pt(II) . . . Pt(II)Interaction in the Model Compound {Pt2[NHCHN(C(CH2)(CH3))]4},” Inorg.Chem. (1996) 35, 7829-7835; Lewis et al., “Oligomerization andTwo-Center Oxidative Addition Reactions of a Dimeric Rhodium(1)Complex,” J. Am. Chem. Soc. (1976) 98, 7461-7463, each of which areincorporated by reference.

[0060] In a preferred embodiment of the invention, the bridgingligand(s) is a compound of formula III

[0061] wherein X and Y are selected from atoms or moieties capable offorming a donative bond to a metal center, and B is a five- orsix-membered ring. The dashed line represents an optional double bond.Preferred bridging ligands include:

[0062] and derivatives thereof. Other preferred bridging ligands arecarboxylates (RCOO⁻), thiocarboxyloic acids (RCSS⁻), pyrophosphate(⁻O₃P—O—PO₃ ⁻), or a compound of the formula

[0063] and derivatives thereof.

[0064] Yet other preferred bridging ligands include:

[0065] where:

[0066] X and Z are selected from the group consisting of C, CR, O, N,NR, S and P;

[0067] Y is selected from the group consisting of C, N, S and P;

[0068] R is H or any organic substituent; and

[0069] N and N′ are hydrocarbon chains having 4-8 members, possiblyincluding heteroatoms.

[0070] The bridging ligand(s) ensures that the two metal centers of thebinuclear emissive material are maintained in close proximity to eachother. This allows the binuclear emissive material to emit from acollective excited state, rather than from single-metal (monomer)species. The two metal centers can be strongly or weakly coupled in theground state. The conditions may lead to very different photophysicalprocesses.

[0071] A binuclear species that is weakly interacting in the groundstate (M-M distance≧3 Å) is likely to have an excimer like excitedstate. Generally, an excimer is formed when individual lumophores arebound in the excited state but not bound in the ground state. An excimeris a dimer with an excited state wavefunction that extends over twomolecules or constituent species. For the purposes of the presentinvention, a “constituent species” refers to an individual metalcomplex, i.e., a metal center and the ligands to which it is attached.For the binuclear emissive materials of the present invention, the twometal complexes that comprises the excimer are held in relatively closeproximity by virtue of the bridging ligands. The excited statewavefunction in this system extends over both metal complexes andgenerally leads to a marked decrease in the internuclear spacing. Whenthe excited state relaxes the two parts of the molecule repel each otherand the system returns to the higher internuclear separation found inthe ground state. This weakly interacting system does not represent atrue excimer, since the two constituent species can not completelydissociate, due the constraints of the bridging ligands. Thephotophysics is an excimer-like process however, i.e. excitation of oneof the metal complexes, extension of the excited state wave function toboth metal complexes, contraction, relaxation (emitting light) andfinally, expansion.

[0072] A binuclear species having strongly interacting metal complexesmay exhibit different photophysics. In this case, the ground stateconfiguration may involve the formation of a M-M bonding orbitals. Ford⁸ metal complexes, this bonding orbital involves the contribution oftwo electrons from each metal center, forming filled σ bonding and σantibonding (σ*) orbitals, leading to a net bond order for thisinteraction of 0. This bonding picture has been described previously andis well known to those skilled in the art. See, Siu-Wai Lai et al.,“Probing d 8-d 8 Interactions in Luminescent Mono- and BinuclearCyclometalated Platinum(II) Complexes of 6-Phenyl-2,2′-bipyridines,”Inorg. Chem. (1999) 38, 4046-4055; Mann et al., “Characterization ofOligomers of Tetrakis(phenyl isocyanide)rhodium(I) in AcetonitrileSolution,” J . Am. Chem. Soc. (1975) 97, 3553-3555, each of which areincorporated by reference.

[0073] In the ground state, the highest filled orbital is generally theσ* orbital. The photophysics for this situation involves the promotionof an electron from the M-M σ* orbital to a π* orbital of the ligand ora higher lying M-M bonding orbital.

[0074] When the accepting orbital is the π* orbital, the transition isreferred to as an MMLCT (metal-metal-to-ligand-charge-transfer). The π*orbital is the same state involved in the MLCT transition of a monomericversion of the binuclear complex, and is generally associated with a“photoactive ligand.” Some contraction of the M-M distance is expectedin the MMLCT excited state, since the σ* orbital is depopulated, but thenature of the transition is very different than the excimer-liketransition described for the weakly interacting system. See, Novozhilovaet al., “Theoretical Analysis of the Triplet Excited State of the[Pt2(H2P2O5)4]4- Ion and Comparison with Time-Resolved X-ray andSpectroscopic Results,” J . Am. Chem. Soc. (2003) 125, 1079-1087; Riceet al., “Electronic Absorption and Emission Spectra of BinuclearPlatinum(I1) Complexes. Characterization of the Lowest Singlet andTriplet Excited States of Pt2(H2P205)44-,” J. Am. Chem. Soc. (1983) 105,4571-4575, each of which is incorporated by reference.

[0075] While excitation of the weakly interacting system is the same asa monomeric version, the MMLCT gives rise to a new absorption for the σ*to π* transition, which is not present in the absorption spectrum of themonomeric complex. For example, this new band is seen in the excitationspectra of the binuclear Pt complex (F₂ppy)₂Pt₂(SPy)₂ at 500 nm (seeFIG. 4).

[0076] A binuclear material may have some degree of metal-metal bondingthat occurs in the ground state. Practically, it can be difficult todetermine whether the constituent species comprising the binuclearemitter are directly bound in the ground state or not, when doped intomolecular thin films, of the type used for the fabrication of OLEDs. Itmay be the case for some emitters that the truth is somewhere betweenthe extremes. For example, the constituent species comprising thebinuclear emitter may have a weak metal-metal bond in the ground state,but in the excited state the bond shortens and the species becomesstrongly bound. In this case, the emitter is not a “true” excimer, asthe constituent species are bound in the ground state. The constituentspecies may well be involved in both π-π stacking and metal-metalinteractions in the doped films, leading to either excimer or MMLCTexcited states. Thus, the term “excimer” as used herein may in somecases refer to constituent species having strongly bound excited statesand weakly bound ground states.

[0077] The excimer energy is lower than that of an exciton localized oneither of the two constituent species that make it up and its emissionis typically relatively broad. Since excimers lack a bound ground state,they provide a unique solution to the achievement of efficient energytransfer from the charge-carrying host matrix to the light emittingcenters. Indeed, for the case of two emitting materials, use of anexcimer prohibits energy transfer between the two emitters, eliminatingcomplicated intermolecular interactions, which make color balancingusing multiple dopants problematic. For a review of the properties ofexcimers and excitons see Andrew Gilbert and Jim Baggott, Essentials ofMolecular Photochemistry, 1991, CRC Press, Boston, pp. 145-167.

[0078] The photoactive ligand is referred to as photoactive because itis believed that it contributes to the photoactive properties of theemissive material by providing a π* orbital for an electron. Whether anelectron is ionized from a ligand-based π* orbital, or moves from ametal-based orbital to the ligand-based orbital, the ligand isconsidered photoactive. The photoactive ligand may be bidentate ortridentate, wherein the terms bidentate and tridentate refer to thenumber of bonds the ligand has to a single metal center. For thephotoactive ligand, preferably at least one of the bonds to the metalcenter will be a carbon-metal bond. In a preferred embodiment of theinvention, the photoactive ligands comprise one or more aromatic rings.In some embodiments of the present invention, a first photoactive ligandcoordinated to a first metal center and a second photoactive ligandcoordinated to a second metal center are held in proximity by thebridging ligand(s) and the first and second metal centers allowing for aπ-π interaction between the first and second photoactive ligands. Anysuitable photoactive ligand may be used. In some embodiments, the firstand second photoactive ligands may have the same structure.

[0079] In one embodiment of the present invention, the binuclearemissive compound comprises two metal centers, wherein each metal centeris bound to a tridentate photoactive ligand and to one of the bindingsites of a bridging ligand to give a compound of formula I

[0080] wherein A is a tridentate photoactive ligand, L is a bridgingligand and each M is a metal center. In this embodiment the metalcenters each have a square planar configuration. The photoactivetridentate ligand, A, is bound to the metal center through three bonds,at least one of which is a carbon-metal bond and the remaining bonds tothe metal being donative (heteroatom-metal) bonds. Preferred tridentateligands are tricyclic aromatic compounds. In one embodiment of theinvention, A is a tridentate photoactive ligand of the formula II_(a)

[0081] wherein Ar₁ is a five or six membered azacyclic ring, wherein thering has a nitrogen atom at the 2-position that is capable of forming adonative bond to the metal center. The 2-position is defined herein asthe position adjacent in the ring to the bond to the central pyridinering. In a preferred embodiment, Ar₁ is pyridine or a substitutedpyridine and Ar₂ is phenyl or a substituted phenyl.

[0082] In another embodiment of the invention, A is a tridentatephotoactive ligand of the formula II_(b)

[0083] wherein Ar₃ and Ar₄ are independently selected five or sixmembered azacyclic rings, wherein each ring has a nitrogen atom at the2-position that is capable of forming a donative bond to the metalcenter. In a preferred embodiment, one of the rings Ar₃ and Ar₄ ispyridine or a substituted pyridine. In a particularly preferredembodiment, both Ar₃ and Ar₄ are pyridine or a substituted pyridine.

[0084] In one embodiment of the present invention, the binuclearemissive compound comprises two metal centers, wherein each metal centeris bound to a bidentate photoactive ligand and to two bridging ligandsto give a compound of formula III:

[0085] wherein A′ is a bidentate photoactive ligand, L is a bridgingligand and M is a metal center. The bidentate photoactive ligand, A′,has one metal-carbon bond and one donative (heteroatom-metal) bond, andcan be selected from a wide variety known to the art. In a particularlypreferred embodiment, A′ is selected from 2-phenylpyridine andderivatives thereof. Many preferred bidentate photoactive ligandsinclude the following partial structure, coordinated with the metal, soas to form a cyclometallated organometallic compound such as disclosedin U.S. 2003-0017361, which is incorporated in its entirety byreference, as shown:

[0086] M may be any suitable metal, for example a d7, d8 or d9 metal,and the dotted lines represent bonds to the rest of the photoactiveligand.

[0087] In one embodiment of the invention, the binuclear emissivecompound comprises two metal centers bound by bridging ligands. Thisembodiment may not have any photoactive ligands. For example, the twometal centers may be bound by four bridging ligands to give a compoundof the formula IV:

[0088] wherein M is a metal center and L is a bridging ligand.

[0089] Where a compound having two metal centers, including binuclearcompounds, does not have any photoactive ligands, light emission mayoccur by a mechanism that does not involve a π* orbital associated witha photoactive ligand. Specifically, the HOMO of a binuclear compoundthat does not have any photoactive ligands may also be a σ* orbital,composed predominantly of metal d_(z2) orbitals. The LUMO in theunexcited state may be a σ_(p) orbital (σ bonding orbital formed byoverlap of high lying metal p_(z) orbitals), such that an excited statemay occur when an electron moves from the σ* orbital to the σ_(p)orbital. Such a transition may reduce the distance between the two metalcenters for two reasons, first because the anti-bonding σ* orbitalbecomes depopulated and second because the bonding σ_(p) orbital becomespopulated. Emission may occur by relaxation of an electron from theσ_(p) orbital to the σ* orbital. This type of emission is expected to bephosphoresecent.

[0090] Generally, a molecule having a photoactive ligand will have aLUMO associated with a π* orbital, not the σ_(p) orbital. Many previousefforts to obtain phosphorescent blue-emitting materials involved tuningthe difference between a π* orbital associated with a photoactive ligandand a σ_(p) orbital. Embodiments of the present invention allow bluematerials to be obtained by designing a binuclear or multinuclearmolecule without a photoactive ligand, such that the LUMO is no longer aπ* orbital. Rather, the LUMO is a σ_(p) orbital, and the emissionspectra is determined by the energy difference between the σ* and σ_(p)orbitals. This energy difference may be tuned in several ways. First,the distance between the metal centers may be controlled throughselection of the bridging ligands. This distance has a strong effect onthe energy difference between the σ* and σ_(p) orbitals, and hence theemission spectra. All else being equal, smaller distances will generallyresult in lower energies and red-shifted emission, while greaterdistances will result in higher energies and blue-shifted emission.Second, the bridging ligands may be selected to tune the electronicstructure of the metal centers. For example, the σ* and σ_(p) orbitalsin a binuclear molecule are the result of an interaction between the dzorbitals of the individual metals centers. Some bridging ligands maysignificantly influence the energies of metal d and p orbitals thoughligand field interactions. Such ligand field “tuning” of orbitalenergies may lower or raise the energies of the individual metal d and porbitals, depending on the nature of the bridging ligand and the bondingarrangement. Thus, it is possible that such ligand field effects couldincrease the energy difference between the atomic orbitals of the twometal centers and increase the energy difference between the σ* orbitaland the σ_(p) orbital, thereby blue-shifting the emission spectrum ofthe molecule.

[0091] In a preferred embodiment, a binuclear emissive material has twometal centers bound to each other by four bridging ligands, with nophotoactive ligands. The metal centers preferably have four coordinationsites. Pt is a preferred metal for the metal centers, because it hasfour coplanar coordination sites.

[0092] Preferably, the metal centers of a binuclear emissive materialhave at least 3 coordination sites, and more preferably at least 4coordination sites. It is believed that materials having metal centerswith 3 or 4 coordination sites tend to be more stable than materialshaving fewer coordination sites. It is believed that metal centershaving at least 4 coordination sites may tend to be particularly stable.A metal having exactly 4 coordination sites in a coplaner arrangement,such as Pt, may lead to particularly stable binuclear molecules.

[0093] The compounds of embodiments of the present invention comprisetwo metal centers. The metals may be selected from the heavy metals withan atomic weight greater than 40. The preferred electronic configurationof the metal center has eight d electrons (e.g. Pt(II), Pd(II), Ni(II),Ir(I), Rh(I), Ag(III), Au(III), etc.), but the invention is not limitedto these metals or oxidation states. These metal centers are referred toas “d8” metal centers. d8 metal centers are preferred because there isgenerally a strong interaction between two d8 metal centers, even thoughthere is no bond in the ground state. Pt is a particularly preferred d8metal center. Other electronic configurations that may be used includemetal centers having 7 d electrons (“d7” metal centers), and metalcenters having 9 d electrons (“d9” metal centers). d10 metal centers arenot preferred, because they generally have a long interaction and nobond in the ground state. In some embodiments, a binuclear complex maybe formed from two metals having a different number of d electrons,i.e., a d7 metal may be paired with a d8 metal. Preferably, the twometals have the same number of d electrons. Most preferably, for ease offabrication, the two metal centers of a binuclear complex are the samemetal.

[0094] In some embodiments, an emissive material is a binuclearcharge-neutral compound. Charge neutral compounds are preferred for someapplications because they are easier to sublime and vacuum deposit, suchthat device fabrication by certain methods is facilitated. Compoundsthat are not charge-neutral may be used in other embodiments, butsublimation of such compounds may be difficult such that solutionprocessing is preferred for such compounds.

[0095] It is understood that the various embodiments described hereinare by way of example only, and are not intended to limit the scope ofthe invention. For example, many of the materials and structuresdescribed herein may be substituted with other materials and structureswithout deviating from the spirit of the invention. It is understoodthat various theories as to why the invention works are not intended tobe limiting. For example, theories relating to charge transfer are notintended to be limiting.

[0096] Material Definitions:

[0097] As used herein, abbreviations refer to materials as follows: CBP:4,4′-N,N-dicarbazole-biphenyl m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine Alq₃: 8-tris-hydroxyquinolinealuminum Bphen: 4,7-diphenyl-1,10-phenanthroline n-BPhen: n-dopedBPhen(doped with lithium) F₄-TCNQ: tetrafluoro-tetracyano-quinodimethanep-MTDATA: p-doped m-MTDATA(doped with F₄-TCNQ) Ir(ppy)₃:tris(2-phenylpyridine)-iridium Ir(ppz)₃:tris(1-phenylpyrazoloto,N,C(2′)iridium(III) BCP:2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline TAZ:3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole CuPc: copperphthalocyanine. ITO: indium tin oxide NPD: naphthyl-phenyl-diamine TPD:N,N′-bis(3-methylphenyl)-N,N′-bis-(phenyl)- benzidine BAlq:aluminum(III)bis(2-methyl-8-quinolinato)4- phenylphenolate mCP:1,3-N,N-dicarbazole-benzene DCM:4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2- methyl)-4H-pyran DMQA:N,N′-dimethylquinacridone PEDOT:PSS: an aqueous dispersion of poly(3,4-ethylenedioxythiophene) with polystyrenesulfonate(PSS) FPt:Platinum(II)(2-(4-,6-difluorophenyl)pyridinato-N,C²)(2,4-pentanedionato-O,O) FPtdpm:Platinum(II)(2-(4-,6-difluorophenyl)pyridinato-N,C²)(2,2,6,6-tetramethyl-3,5-heptanedionato-O,O) (F₂ppy)₂Pt₂(SPy)₂:di-Platinum(II),bis(2-(4-,6- difluorophenyl)pyridinato-N,C²)bis[μ-(2-pyridinethionato-N1:S2)] Pt₂Spy₄:di-Platinum(II),tetrakis[μ-(2-pyridinethionato- N1:S2)] Spy:2-thiopyridine

[0098] Experimental:

[0099] Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

EXAMPLE 1

[0100] 2-Mercaptopyridine (0.53 g, 4.80 mmol) was added to a methanolicsolution of 4,6-dfppyPtdimer (2.00 g, 2.38 mmol), as described in Brookset al., “Synthesis and Characterization of Phosphorescent CyclometalatedPlatinum Complexes,” Inorganic Chemistry, 2002, 41(12), 3055-3066, whichis incorporated by reference in its entirety. Potassium carbonate (0.50g) was added to the solution and heated to 60° C. for 18 hours. Thesolution was then cooled and the solvent was removed under reducedpressure. The crude product was dissolved in acetone and passed througha silica gel column with acetone as the eluent. The solvent was onceagain removed under reduced pressure and the product was recrystalisedfrom methanol to give an 82% yield of (F₂ppy)₂Pt₂(SPy)₂ as a redcrystalline compound.

EXAMPLE 2

[0101] A glass substrate was prepared by washing with detergent andrinsing with deionized water followed by acetone. The glass was thendried under a stream of nitrogen and then placed in an ozone oven for 10minutes. A 100 ml solution of (F₂ppy)₂Pt₂(SPy)₂/CBP was prepared(solution A) by dissolving 5.00 mg (F₂ppy)₂Pt₂(SPy)₂ and 100 mg of CBPin toluene in a 100 ml volumetric flask. A 100 ml solution of FPt/CBPwas prepared (solution B) by dissolving 5.00 mg FPt and 100 mg of CBP intoluene in a 100 ml volumetric flask. Two thin films were prepared fromthese solutions. Solution A was spincoated on the glass substrate at40,000 rpm for 40 seconds to give the (F₂ppy)₂Pt₂(SPy)₂ thin film.Solution B was spincoated on another glass substrate at 40,000 rpm for40 seconds to give the FPt thin film.

[0102] The thin film of (F₂ppy)₂Pt₂(SPy)₂ doped in CBP was excited withtwo spectra, one peaking at 370 nm and the other peaking at 500 nm.These two spectra were selected to roughly coincide with the absorptionspectra of CBP and (F₂ppy)₂Pt₂(SPy)₂, respectively. The resultant PLspectra are shown in FIG. 3. Plot 310 shows the PL spectra forexcitation at 370 nm. Plot 320 shows the PL spectra for excitation at500 nm. The plot for excitation at 370 nm shows that energy absorbed bythe CBP may be transferred to the (F₂ppy)₂Pt₂(SPy)₂ and emitted aslight.

[0103]FIG. 4 shows the excitation spectra of the thin film of(F₂ppy)₂Pt₂(SPy)₂ doped at 5% in CBP with emission set at 605 nm. Plot420 is based on the same data as plot 410, but the values have beenmultiplied by 20 so that more detail can be seen. The peak around 350nm, most clearly visible in plot 410, is due to absorption by CBP. Thepeak around 500 nm, most clearly visible in plot 420, is due toabsorption by (F₂ppy)₂Pt₂(SPy)₂, and demonstrates an interaction betweenthe two Pt metal centers in the ground state.

[0104]FIG. 5 shows a comparison of the emission spectra of FPt, FPtdpmand (F₂ppy)₂Pt₂(SPy)₂. Photoluminescent films of FPt, FPtdpm and(F₂ppy)₂Pt₂(SPy)₂ in CBP, each doped at 5%, were prepared by spincoating as described above and were excited at 370 nm. The PL spectrafor FPt, FPtdpm and (F₂ppy)₂Pt₂(SPy)₂, respectively, are shown as plots510, 520 and 530.

EXAMPLE 3

[0105] Organic light emitting devices were grown on a glass substratepre-coated with a ˜100 nm thick layer of indium-tin-oxide (ITO) having asheet resistance of ˜20 Ω/□. Substrates were degreased with solvents andthen cleaned by exposure to UV-ozone ambient for 10 minutes. Aftercleaning, the substrates were immediately loaded into a thermalevaporation system operating at a base pressure of ˜1×10⁻⁶ Torr. Severaldifferent device structures were fabricated, as illustrated in FIGS. 6and 8. First, a 400-Å-thick4-4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD) hole transportlayer (HTL) were deposited. In some devices, a 200 Å thick layer of mCPwas deposited as an electron blocking layer (EBL). Next, 9%(F₂ppy)₂Pt₂(SPy)₂ (by weight) was codeposited with either mCP or CBP toform the 300-Å-thick emissive layer. In some devices, a 120 Å thicklayer of BCP was deposited as an electron blocking layer (EBL). Finally,a 350-Å-thick electron transport layer (ETL) consisting of zirconium(IV) tetras (8-hydroxyquinoline) (Zrq₄) was deposited. Device cathodesconsisting of a 10-Å-thick layer of LiF followed by a 1000-Å-thick layerof aluminum were deposited trough a shadow mask. The devices active areawas 2×2 mm². The following four structures were fabricated:

[0106] Structure 1: ITO/NPD/(F₂ppy)₂Pt₂(SPy)₂:CBP/ZrQ₄/LiF:Al

[0107] Structure 2: ITO/NPD/(F₂ppy)₂Pt₂(SPy)₂:CBP/BCP/ZrQ₄/LiF:Al

[0108] Structure 3: ITO/NPD/(F₂ppy)₂Pt₂(SPy)₂:mCP/ZrQ₄/LiF:Al

[0109] Structure 4: ITO/NPD/mCP/(F₂ppy)₂Pt₂(SPy)₂:mCP/ZrQ₄/LiF:Al

[0110]FIG. 6 show a schematic representation of devices havingstructures 1 and 2. Structure 1 has an anode 615, a hole transport layer625, an emissive layer 635, an electron transport layer 645, and acathode 660. Structure 2 is the same as structure 1, but has anadditional hole blocking layer 640 disposed between emissive layer 635and electron transport layer 645. The materials and thicknesses of thevarious layers were as indicated in the previous paragraphs.

[0111]FIG. 7 shows plots of quantum efficiency against current densityfor devices having Structures 1 and 2. Plot 710 shows data for structure1, and plot 720 shows data for structure 2. Both devices show a maximumquantum efficiency of about 6.0%.

[0112]FIG. 8 shows plots of current density vs. voltage for deviceshaving Sructures 1 and 2. Plot 810 shows data for structure 1, and plot820 shows data for structure 2.

[0113]FIG. 9 shows plots of electroluminescent spectra for deviceshaving Structures 1 and 2. Plot 910 shows data for structure 1, and plot920 shows data for structure 2.

[0114]FIG. 10 shows plots of brightness vs voltage for the deviceshaving structures 1 and 2. Plot 1010 shows data for structure 1, andplot 1020 shows data for structure 2. Structure 1 shows a higherbrightness at 6 volts of about 100 Cd/m².

[0115]FIG. 11 show a schematic representation of devices havingstructures 3 and 4. Structure 3 has an anode 1115, a hole transportlayer 1125, an emissive layer 1135, an electron transport layer 1145,and a cathode 1160. Structure 4 is the same as structure 3, but has anadditional electron blocking layer 1130 disposed between hole transportlayer 1125 and emissive layer 1135. The materials and thicknesses of thevarious layers were as indicated in the previous paragraphs.

[0116]FIG. 12 shows plots of quantum efficiency against current densityfor devices having the structures NPD/mCP/EL/Zrq₄ and NPD/EL/Zrq₄(Structures 4 and 3). Plot 1210 shows data for structure 3, and plot1220 shows data for structure 4. Both devices show a maximum quantumefficiency of about 3.1%.

[0117]FIG. 13 shows plots of current density vs. voltage for deviceshaving the structures NPD/mCP/EL/Zrq₄ and NPD/EL/Zrq₄ (Structures 4 and3). Plot 1310 shows data for structure 3, and plot 1320 shows data forstructure 4.

[0118]FIG. 14 shows plots of electroluminescent spectra for deviceshaving the structure NPD/mCP/EL/Zrq₄ and NPD/EL/Zrq₄ (Structures 4 and3). Plot 1410 shows data for structure 3, and plot 1420 shows data forstructure 4.

[0119]FIG. 15 shows plots of brightness vs voltage for the deviceshaving the structure NPD/mCP/EL/Zrq₄ and NPD/EL/Zrq₄ (Structures 4 and3). Plot 1510 shows data for structure 3, and plot 1520 shows data forstructure 4.

[0120]FIG. 16 shows the chemical structure of FPt, FPtdpm, and(F₂ppy)₂Pt₂(SPy)₂.

[0121]FIG. 17 shows the structure of (F₂ppy)₂Pt₂(SPy)₂ as determined byX-ray crystallography.

[0122] Pt2Spy4(n) and Spy(n) were prepared as described in Umakoshi etal., “Binuclear Platinum(II) and -(III) Complexes of Pyridine-2-thioland Its 4-Methyl Analogue, Synthesis, Structure, and Electrochemistry,”Inorg. Chem. 1987, 26, 3551-3556, which is incorporated by reference inits entirety.

[0123]FIG. 18 shows PL emission spectra for Pt2Spy₄ at a concentrationof less than 1 nM in 2-methyl-tetra-hydro-furan (2-methyl-THF). Plot1810 shows an emission peak for 2-methyl-THF, and plot 1220 shows anemission peak for Pt₂Spy₄. The peak of plot 1820 is aroud 400 nm, andthe tail is significantly attenuated at 470 nm, demonstrating that it ispossible to achieve deep blue emission with a binuclear material havingno photoactive ligands.

[0124]FIG. 19 shows absorption spectra for Pt₂Spy₄ and Spy in solution.Plot 1910 shows the absorption spectra for Spy. Plot 1920 shows theabsorption spectra for Pt₂Spy₄.

[0125] While the present invention is described with respect toparticular examples and preferred embodiments, it is understood that thepresent invention is not limited to these examples and embodiments. Thepresent invention as claimed therefore includes variations from theparticular examples and preferred embodiments described herein, as willbe apparent to one of skill in the art

What is claimed is:
 1. An organic light emitting device, comprising: an anode; a cathode; and an emissive layer disposed between and electrically connected to the anode and the cathode, the emissive layer further comprising an emissive material comprising: a first metal center selected from the group consisting of d7, d8 and d9 metals; a second metal center selected from the group consisting of d7, d8 and d9 metals; and a bridging ligand coordinated to the first metal center and to the second metal center.
 2. The device of claim 1, wherein the first metal center and the second metal center are both selected from the group consisting of d8 metals.
 3. The device of claim 2, wherein the distance between the first metal center and second metal center is such that there is significant overlap between a d8 orbital of the first metal center and a d8 orbital of the second metal center.
 4. The device of claim 2, wherein the first metal center and the second metal center are both the same metal.
 5. The device of claim 4, wherein the first metal center and the second metal center are both Pt.
 6. The device of claim 1, wherein the emissive material is a binuclear material that comprises: a first metal center; a second metal center; and four bridging ligands, wherein each bridging ligand is coordinated to the first metal center and to the second metal center.
 7. The device of claim 1, wherein the emissive material is a binuclear material that comprises: a first metal center; a second metal center; two bridging ligands, wherein each bridging ligand is coordinated to the first metal center and to the second metal center; a first bidentate photoactive ligand bound to the first metal center, and a second bidentate photoactive ligand bound to the second metal center.
 8. The device of claim 7, wherein the first photoactive ligand and the second photoactive ligand are held in proximity by the bridging ligand and the first and second metal centers allowing for a significant π-π interaction between the first and second photoactive ligands.
 9. The device of claim 8, wherein the first and second photoactive ligands have the same structure.
 10. The device of claim 1, wherein the emissive material is a binuclear material that comprises: a first metal center; a second metal center; one bridging ligand, wherein the bridging ligand is coordinated to the first metal center and to the second metal center; a first tridentate photoactive ligand bound to the first metal center, and a second tridentate photoactive ligand bound to the second metal center.
 11. The device of claim 10, wherein the first photoactive ligand and the second photoactive ligand are held in proximity by the bridging ligand and the first and second metal centers allowing for a significant π-π interaction between the first and second photoactive ligands.
 12. The device of claim 11, wherein the first and second photoactive ligands have the same structure.
 13. The device of claim 1, wherein the emissive layer further comprises a host material, and the emissive material is present as a dopant in the host material.
 14. The device of claim 1, wherein the device comprises an anode; a hole transporting layer; an electron transporting layer; a cathode; and an emissive layer, wherein the emissive layer is disposed between the hole transporting layer and the electron transporting layer.
 15. The device of claim 1, wherein the device substantially all of the light emitted from the device results from excimer emission.
 16. The device of claim 1, wherein the emissive molecule is a phosphorescent emissive material.
 17. The device of claim 7, wherein the first and second photoactive ligands have the following partial structure:

wherein M is a metal center.
 18. The device of claim 1, wherein each bridging ligand is selected from the group consisting of carboxylates (RCOO⁻), thiocarboxyloic acids (RCSS⁻), pyrophosphate (⁻O₃P—O—PO₃ ⁻), or a compound of the formula

and derivatives thereof.
 19. The device of claim 1, wherein the bridging ligand is selected from the group consisting of:

where: X and Z are selected from the group consisting of C, CR, O, N, NR, S and P; Y is selected from the group consisting of C, N, S and P; R is H or any organic substituent; and N and N′ are hydrocarbon chains having 4-8 members.
 20. The device of claim 19, wherein N and N′ include heteroatoms.
 21. The device of claim 1, wherein the emissive molecule has the structure:


22. The device of claim 1, wherein the distance between the first metal center and the second metal center is less than about 4 angstroms.
 23. The device of claim 22, wherein the distance between the first metal center and the second metal center is about 2.5 angstroms to about 2.8 angstroms.
 24. The device of claim 1, wherein the emissive material does not include a photoactive ligand.
 25. An organic light emitting device, comprising: an anode; a cathode; and an emissive layer disposed between and electrically connected to the anode and the cathode, the emissive layer further comprising a binuclear emissive material comprising: a first metal center having a coordination number of at least 3; a second metal center having a coordination number of at least 3; and a bridging ligand coordinated to the first metal center and to the second metal center.
 26. The device of claim 25, wherein the first metal center and the second metal center both have a coordination number of at least
 4. 27. The device of claim 26, wherein the first metal center and the second metal center both have a coordination number of
 4. 28. The device of claim 25, wherein the emissive material does not include a photoactive ligand.
 29. The device of claim 25, wherein: the first metal center and the second metal center have the same coordination number; and the emissive material further comprises a plurality of bridging ligands coordinated to the first metal center and the second metal center, such that the number of bridging ligands is equal to the coordination number of the first metal center.
 30. An organic light emitting device, comprising: an anode; a cathode; and an emissive layer disposed between and electrically connected to the anode and the cathode, the emissive layer further comprising a binuclear emissive material comprising: a first metal center; a second metal center; a bridging ligand coordinated to the first metal center and to the second metal center; a first photoactive ligand coordinated to the first metal center; a second photoactive ligand coordinated to the second metal center.
 31. The device of claim 30, wherein the first and second photoactive ligands have the following partial structure:

wherein M is a d7, d8 or d9 metal.
 32. The device of claim 30, wherein each bridging ligand is selected from the group consisting of carboxylates (RCOO⁻), thiocarboxyloic acids (RCSS⁻), pyrophosphate (⁻O₃P—O—PO₃ ⁻), or a compound of the formula

and derivatives thereof.
 33. The device of claim 30, wherein the bridging ligand is selected from the group consisting of:

where: X and Z are selected from the group consisting of C, CR, O, N, NR, S and P; Y is selected from the group consisting of C, N, S and P; R is H or any organic substituent; and N and N′ are hydrocarbon chains having 4-8 members.
 34. The device of claim 33, wherein N and N′ include heteroatoms.
 35. An organic light emitting device, comprising: an anode; a cathode; and an emissive layer disposed between and electrically connected to the anode and the cathode, the emissive layer further comprising a binuclear emissive material comprising: a first metal center; a second metal center; and a bridging ligand coordinated to the first metal center and to the second metal center; wherein the binuclear emissive material is charge neutral.
 36. An organic light emitting device, comprising: an anode; a cathode; and an emissive layer disposed between and electrically connected to the anode and the cathode, the emissive layer further comprising a binuclear emissive material comprising: a first metal center; a second metal center; and a bridging ligand coordinated to the first metal center and to the second metal center; wherein the metal centers are held in a co-facial configuration by the bridging ligand.
 37. The device of claim 36, wherein the first and second metal centers each have a square planar configuration.
 38. An organic light emitting device, comprising: an anode; a cathode; and an emissive layer disposed between and electrically connected to the anode and the cathode, the emissive layer further comprising an emissive material comprising: a first metal center selected from the group consisting of metals having an atomic number greater than or equal to 40; a second metal center selected from the group consisting of metals having an atomic number greater than or equal to 40; and a bridging ligand coordinated to the first metal center and to the second metal center. 